1. Field of the Invention
The present invention relates to a laminate comprising a substrate such as of a metal and a polyester layer, and to a seamless can formed by using this laminate. More specifically, the invention relates to a laminate featuring high productivity, having uniform and intimately adhered layer, and exhibiting excellent shock resistance (dent resistance) and excellent dent resistance (heat/dent resistance) after having received thermal hysteresis at high temperatures, and to a seamless can.
2. Prior Art
A side seamless can has heretofore been obtained by subjecting a metal blank such as aluminum plate, tin plate or tin-free steel plate to at least one stage of draw working between a drawing die and a punch to form a cup comprising a barrel without side seam and a bottom integrally connected to the barrel without forming seam and, then as required, subjecting the barrel to the ironing working between an ironing punch and a die to decrease the thickness of the container barrel. It has also been known already to decrease the thickness of the side wall relying upon the bend-elongation at the corner of curvature of the redrawing die instead of effecting the ironing working (Japanese Patent Publication No. 501442/1981).
As a method of applying an organic coating material onto a side seamless can, furthermore, there has been known a method of laminating a resin film on a metal blank prior to the forming in addition to a method of applying an organic coating material onto the widely used cans after the forming. According to Japanese Patent Publication No. 34580/1984, a polyester film derived from a terephthalic acid and a tetramethylene glycol is laminated on a metal blank. It has further been known to produce a redrawn can based on the bend-elongation, by using a metal plate coated, for example, with a vinyl organosol, epoxy, phenolics, polyester, acrylic resin, etc.
A variety of methods have also been proposed for producing polyester-coated metal plates. For example, Japanese Patent Publication No. 34580/1984 discloses a metal blank on which is laminated a polyester film derived from a terephthalic acid and a tetramethylene glycol, for producing the cans.
Japanese Laid-Open Patent Publication No. 4229/1993 discloses a film composed of a polyethylene terephthalate having a trace of biaxial orientation on the surface thereof, and Japanese Laid-Open Patent Publication No. 172556/1994 proposes a polyester film having a intrinsic viscosity [.eta.] of not smaller than 0.75 that is to be laminated on a metal.
Furthermore, Japanese Laid-Open Patent Publication No. 101930/1991 discloses a coated metal plate for drawn cans, comprising a laminate of a metal plate, a polyester film layer composed chiefly of an ethylene terephthalate unit and, as required, an adhesive primer layer interposed between the metal plate and the polyester film, the polyester layer having an X-ray diffraction intensity as defined by the formula, EQU Rx=IA/IB
where IA is an X-ray diffraction intensity on a diffraction plane having a spacing of about 0.34 nm (CuK.alpha. X-ray diffraction angle of 24.degree. to 28.degree.) in parallel with the surface of the polyester film, and IB is an X-ray diffraction intensity on a diffraction plane having a spacing of about 0.39 nm (CuK.alpha. X-ray diffraction angle of 21.5.degree. to 24.degree.) in parallel with the surface of the polyester film, PA1 wherein .eta..sub.12.2 is a melt viscosity at an extrusion temperature of the polyester at a shearing rate of 12.2 sec.sup.-1, and .eta..sub.1216 is a melt viscosity at an extrusion temperature of the polyester at a shearing rate of 1216 sec.sup.-1, PA1 1. The resin layer has a melt tension of from 0.2 to 1.5 grams at a temperature of the melt-extrusion; PA1 2. The resin layer has a dieswell of from 1.3 to 2.0 at the temperature of the melt-extrusion; and PA1 3. The resin layer has a degree of polydispersion (d) defined by the following formula (2), EQU d=Mw/Mn (2) PA1 wherein Mw is a weight average molecular weight, and Mn is a number average molecular weight, PA1 Hu: quantity of heat of fusion of crystalline polyester composed chiefly of an ethylene terephthalate unit, 9200 (J/mol), PA1 R: gas constant, 8.314 (J/(mol.multidot.K)), PA1 Tm: melting point (K) of the blend, PA1 Tm0: melting point (K) of the crystalline polyester composed chiefly of the ethylene terephthalate unit,
of from 0.1 to 15, and an anisotropic index of in-plane orientation of crystals of smaller than 30. There has further been disclosed a thin draw-worked can obtained by subjecting the coated metal plate to the drawing and redrawing, and subjecting the side wall of the can barrel to the bend-elongation to decrease the thickness at the time of redraw working.
According to the above-mentioned prior art, a resin film may be applied to the metal blank of before being formed requiring neither the furnace for firing the coating nor the facility for treating exhaust gases of coating material like the ordinarily employed coating processing, giving such advantages that the air is not polluted and the can after formed needs not be coated with the coating material still leaving, however, a room for improvement from the standpoint of forming a film and extrusion-coating onto the metal base. When the laminate is used for producing cans, furthermore, there still is a room for improvement in regard to formability into cans, properties of the cans and, particularly, shock resistance (dent resistance), shock resistance after thermal hysteresis, corrosion resistance, wrap-seaming property and sealing property.
That is, in the general metal-resin laminate, it is widely accepted practice to extrude and apply the resin onto the metal base. If this method of extrusion coating can be adapted even to the polyester, then, it can be expected to decrease the work for forming films and for effecting the drawing and, hence, to decrease the cost therefor.
However, a metal-polyester laminate and, particularly, a laminate for forming seamless containers is subject to be put to heat treatments during the steps of production causing the undrawn or unoriented polyester to be thermally crystallized (whitening, formation of lamella), whereby the coating becomes brittle to impair the workability. If the polyester of the laminate is maintained in the molecularly oriented state, then, excessive thermal crystallization is not formed during the heat treatment. This is why the molecularly oriented polyester film is used for the laminate.
As a method of producing a laminate by extruding and laminating a thermoplastic resin onto a base material, furthermore, there has been known a so-called T-die method by using an extruder and a T-die. When it is attempted to laminate the polyester resin based on the T-die method, however, the flow loses stability in the extruder and in the die, and there takes place a so-called draw resonance phenomenon in which the selvage deviates in a zigzag manner due to lack of tension after the resin has come out from the T-die, making it difficult to obtain a uniform film thickness. Besides, both ends of the deviated selvage must be trimmed hindering the yield. These phenomena occurs particularly when the speed for taking up the resin is high making it very difficult to laminate the polyester resin at a high speed.
Moreover, the polyester resin loses viscosity as it is deteriorated with heat. In an extreme case, therefore, the polyester resin drops in lumps from the T-die.
Similar problem occurs in the stage of forming a film even when an undrawn cast film is used or a biaxially drawn film is used for forming the laminate.
A practical shock resistance that is required when the laminate is really used for producing canned products may be the so-called dent resistance. This is the property in that the coating stays intimately adhered or maintains a complete coverage even in case the canned products are dented as they fell or came into collision with each other. That is, in case the coating is peeled off or the pinholes or cracks are formed in the coating during the denting testing, the metal elutes out from such portions or leakage occurs through pitting, causing the contents to be no longer preserved.
Next, in the case of the cans for the canned products, the coating inevitably receives the effect of heat treatment. Namely, it is accepted practice to print the indication of contents on the outer surface of the can, and the polyester film is affected by the heat of firing the printing ink. In practically producing the cans, furthermore, the cans are often heated to relax internal stress and stabilize the resin coating, and the effect of heating upon the polyester is not negligible. The polyester tends to be thermally crystallized and deteriorated, i.e., tends to lose the molecular weight upon the heating, whereby the dent resistance decreases, adhesiveness to the metal base decreases, coating property decreases, and workability decreases such as necking workability, wrap-seam workability, etc.